Materials and methods for preparing dimeric growth factors

ABSTRACT

Proteins consisting of, from amino to carboxyl terminus, a first PDGF-D growth factor domain polypeptide, a linker polypeptide, and a second PDGF-D growth factor domain polypeptide, and materials and methods for making the proteins are disclosed. Each of the first and second PDGF-D growth factor domain polypeptides consists of a sequence of amino acid residues as shown in SEQ ID NO:2 or SEQ ID NO:4 from amino acid x to amino acid y, wherein x is an integer from 246 to 258, inclusive, and y is an integer from 365-370, inclusive. The linker polypeptide consists of from 11-40 amino acid residues. The proteins can be used to stimulate the production of bone and/or connective tissue in both humans and non-human animals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 10/365,095, filed Feb. 11, 2003, which claims the benefit under 35 U.S.C. 119(e) of provisional application Serial No. 60/355,882, filed Feb. 11, 2002, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

In multicellular animals, cell growth, differentiation, and migration are controlled by polypeptide growth factors. These growth factors play a role in both normal development and pathogenesis, including the development of solid tumors. Polypeptide growth factors influence cellular events by binding to cell-surface receptors, many of which are tyrosine kinases. Binding initiates a chain of signalling events within the cell, which ultimately results in phenotypic changes, such as cell division, protease production, and cell migration.

Growth factors can be classified into families on the basis of structural similarities. One such family, the PDGF (platelet derived growth factor) family, is characterized by a dimeric structure stabilized by disulfide bonds. This family includes the PDGFs, the placental growth factors (P1GFs), and the vascular endothelial growth factors (VEGFs). The PDGFs are a group of disulfide-bonded, dimeric proteins. Four PDGF polypeptide chains have been identified and named A, B, C, and D chain. The A and B chains forms dimers with themselves and each other, resulting in AA, BB, and AB dimers. See, in general, Ross et al., Cell 46:155-169, 1986 and Hart et al., Biochem. 29:166-172, 1990. Recombinant forms of these proteins, including truncated and substitutional variants, are disclosed in U.S. Pat. Nos. 4,801,542; 4,845,075; 4,849,407; 4,889,919; and 5,895,755. Two additional PDGF polypeptides, designated C and D, have been described. See, WIPO Publication WO 00/34474; WIPO Publication WO 00/66736; Bergsten et al., Nature Cell Biol. 3:512-516, 2001; LaRochelle et al., Nature Cell Biol. 3:517-521, 2001; and Uutela et al., Circulation 103:2242-2247, 2001. PDGF-C is also known as “zvegf3” (WO 00/34474), and PDGF-D is also known as “zvegf4” (WO 00/66736).

PDGF-C and PDGF-D have a multidomain structure that comprises an amino-terminal CUB domain and a carboxyl-terminal growth factor domain joined by an interdomain region of approximately 70 amino acid residues. The growth factor domain of PDGF-D, which comprises approximately residues 250-370 of human PDGF-D (SEQ ID NO:2), is characterized by an arrangement of cysteine residues and beta strands that is characteristic of the “cystine knot” structure of the PDGF family. The CUB domain shows sequence homology to CUB domains in the neuropilins (Takagi et al., Neuron 7:295-307, 1991; Soker et al., Cell 92:735-745, 1998), human bone morphogenetic protein-1 (Wozney et al., Science 242:1528-1534, 1988), porcine seminal plasma protein and bovine acidic seminal fluid protein (Romero et al., Nat. Struct. Biol. 4:783-788, 1997), and Xenopus laevis tolloid-like protein (Lin et al., Dev. Growth Differ. 39:43-51, 1997).

PDGF-C and PDGF-D form homodimeric proteins (PDGF-CC and PDGF-DD) that are proteolytically cleaved to produce the active species, in each case a growth factor domain dimer. The active PDGF-DD protein binds to and activates the α/α, β/β and α/β isoforms of the PDGF receptor. PDGF-DD dimers are mitogenic for a variety of mesenchymal cells (Bergsten et al., ibid.; LaRochelle et al., ibid.). In addition, mice infected with a PDGF-D adenovirus construct showed proliferation of endosteal bone (U.S. patent application Ser. No. 09/540,224).

Production of biologically active, recombinant PDGF-DD has been found to be problematic. See, for example, Bergsten et al., ibid. There is a need in the art for materials and methods for producing recombinant PDGF in economically feasible amounts.

DESCRIPTION OF THE INVENTION

Within one aspect of the present invention there is provided a fusion protein consisting of, from amino terminus to carboxyl terminus, a first PDGF-D growth factor domain polypeptide, a linker polypeptide, and a second PDGF-D growth factor domain polypeptide, wherein each of the first and second PDGF-D growth factor domain polypeptides consists of a sequence of amino acid residues as shown in SEQ ID NO:2 or SEQ ID NO:4 from amino acid x to amino acid y, wherein x is an integer from 246 to 258, inclusive, and y is an integer from 365-370, inclusive; wherein the linker polypeptide consists of from 11-40 amino acid residues; and wherein the fusion protein is optionally glycosylated. Within one embodiment of the invention, y is 370. Within other embodiments of the invention, x is 246, 248, or 250. Within another embodiment of the invention, x is 250 and y is 370. Within further embodiments of the invention, the linker polypeptide consists of from 12 to 20 amino acid residues or from 14 to 16 amino acid residues. Within still other embodiments of the invention, the linker polypeptide does not contain Lys or Arg, the linker polypeptide does not contain Cys, or the linker polypeptide does not contain Pro. Within a further embodiment of the invention, the linker polypeptide comprises a proteolytic cleavage site. Within another embodiment of the invention, the first and second PDGF-D growth factor domain polypeptides are joined by at least one interchain disulfide bond.

Within a second aspect of the invention there is provided a polynucleotide encoding a fusion protein as disclosed above. Within one embodiment, the polynucleotide further encodes a secretory peptide operably linked to the fusion protein. Within another embodiment, the polynucleotide is DNA.

Within a third aspect of the invention there is provided an expression vector comprising the following operably linked elements: a transcription promoter; a DNA segment encoding a fusion protein as disclosed above; and a transcription terminator.

Within a fourth aspect of the invention there is provided a cultured cell into which has been introduced an expression vector as disclosed above.

Within a fifth aspect of the invention there is provided a method of making a protein comprising the steps of culturing a cell as disclosed above in a culture medium whereby the DNA segment is expressed and the fusion protein is produced, and recovering the fusion protein. Within one embodiment, the cell is a eukaryotic cell, the DNA segment encodes a secretory peptide operably linked to the fusion protein, and the fusion protein is secreted from the cell and is recovered from the culture medium. Within a related embodiment, the recovered fusion protein comprises at least one disulfide bond joining the first PDGF-D growth factor domain polypeptide to the second PDGF-D growth factor domain polypeptide. Within another embodiment, the linker polypeptide comprises a proteolytic cleavage site and, subsequent to the recovering step, the fusion protein is proteolytically cleaved at the cleavage site. Within a further embodiment, the linker polypeptide comprises a proteolytic cleavage site, the cell produces a protease that cleaves at the cleavage site, and the fusion protein is cleaved by the protease within the cell during secretion. Within an additional embodiment, the cell is a prokaryotic cell.

Within a sixth aspect of the invention there is provided a protein produced according to the method disclosed above.

These and other aspects of the invention will become evident upon reference to the following detailed description of the invention.

As used herein, a “biologically active” PDGF-DD protein is a PDGF-DD protein that binds to cell-surface PDGF receptors (α/α, α/β, or β/β receptors) and thereby stimulates a cellular response such as migration, differentiation, or mitosis.

The phrase “a cultured cell into which has been introduced an expression vector” includes cells that have been physically manipulated to contain the vector, as well as progeny of the manipulated cells when the progeny also contain the vector.

The term “expression vector” is used to denote a DNA molecule, linear or circular, that comprises a segment encoding a polypeptide of interest operably linked to additional segments that provide for its transcription. Such additional segments include promoter and terminator sequences, and may also include one or more origins of replication, one or more selectable markers, an enhancer, a polyadenylation signal, etc. Expression vectors are generally derived from plasmid or viral DNA, or may contain elements of both.

The term “isolated”, when applied to a polynucleotide, denotes that the polynucleotide has been removed from its natural genetic milieu and is thus free of other extraneous or unwanted coding sequences, and is in a form suitable for use within genetically engineered protein production systems. Such isolated molecules are those that are separated from their natural environment and include cDNA and genomic clones. Isolated polynucleotide molecules of the present invention are free of other genes with which they are ordinarily associated, but may include naturally occurring 5′ and 3′ untranslated regions such as promoters and terminators. The identification of associated regions will be evident to one of ordinary skill in the art (see, for example, Dynan and Tijan, Nature 316:774-778, 1985).

An “isolated” polypeptide or protein is a polypeptide or protein that is found in a condition other than its native environment, such as apart from blood and animal tissue. Within one embodiment, the isolated polypeptide or protein is substantially free of other polypeptides or proteins, particularly other polypeptides or proteins of animal origin. Isolated polypeptides or proteins may be provided in a highly purified form, i.e. greater than 95% pure or greater than 99% pure. When used in this context, the term “isolated” does not exclude the presence of the same polypeptide or protein in alternative physical forms, such as dimers or alternatively glycosylated or derivatized forms.

“Operably linked” means that two or more entities are joined together such that they function in concert for their intended purposes. When referring to DNA segments, the phrase indicates, for example, that coding sequences are joined in the correct reading frame, and transcription initiates in the promoter and proceeds through the coding segment(s) to the terminator. When referring to polypeptides, “operably linked” includes both covalently (e.g., by disulfide bonding) and non-covalently (e.g., by hydrogen bonding, hydrophobic interactions, or salt-bridge interactions) linked sequences, wherein the desired function(s) of the sequences are retained.

The term “PDGF-D polypeptide” is used herein to denote a polypeptide comprising the core growth factor domain of a PDGF-D (e.g., residues 258-365 of human zvegf4 (SEQ ID NO:2) or mouse zvegf4 (SEQ ID NO:4)). A PDGF-D polypeptide may further comprise one or more additional amino acids derived from the full-length PDGF-D polypeptide chain or from a heterologous polypeptide. Using methods known in the art, PDGF-D polypeptides can be prepared in a variety of forms, including glycosylated or non-glycosylated, pegylated or non-pegylated, with or without an initial methionine residue, and as fusion polypeptides. PDGF-D polypeptides may be in the form of monomers or disulfide-bonded dimers.

A “polynucleotide” is a single- or double-stranded polymer of deoxyribonucleotide or ribonucleotide bases read from the 5′ to the 3′ end. Polynucleotides include RNA and DNA, and may be isolated from natural sources, synthesized in vitro, or prepared from a combination of natural and synthetic molecules. Sizes of polynucleotides are expressed as base pairs (abbreviated “bp”), nucleotides (“nt”), or kilobases (“kb”). Where the context allows, the latter two terms may describe polynucleotides that are single-stranded or double-stranded. When the term is applied to double-stranded molecules it is used to denote overall length and will be understood to be equivalent to the term “base pairs”. It will be recognized by those skilled in the art that the two strands of a double-stranded polynucleotide may differ slightly in length and that the ends thereof may be staggered as a result of enzymatic cleavage; thus all nucleotides within a double-stranded polynucleotide molecule may not be paired. Such unpaired ends will in general not exceed 20 nt in length.

A “polypeptide” is a polymer of amino acid residues joined by peptide bonds, whether produced naturally or synthetically. Polypeptides of less than about 10 amino acid residues are commonly referred to as “peptides”.

The term “promoter” is used herein for its art-recognized meaning to denote a portion of a gene containing DNA sequences that provide for the binding of RNA polymerase and initiation of transcription. Promoter sequences are commonly, but not always, found in the 5′ non-coding regions of genes.

A “protein” is a macromolecule comprising one or more polypeptide chains. A protein may also comprise non-peptidic components, such as carbohydrate groups. Carbohydrates and other non-peptidic substituents may be added to a protein by the cell in which the protein is produced, and will vary with the type of cell. Proteins are defined herein in terms of their amino acid backbone structures; substituents such as carbohydrate groups are generally not specified, but may be present nonetheless.

A “secretory signal sequence” is a DNA sequence that encodes a polypeptide (a “secretory peptide”) that, as a component of a larger polypeptide, directs the larger polypeptide through a secretory pathway of a cell in which it is synthesized. The larger polypeptide is commonly cleaved to remove the secretory peptide during transit through the secretory pathway.

A “segment” is a portion of a larger molecule (e.g., polynucleotide or polypeptide) having specified attributes. For example, a DNA segment encoding a specified polypeptide is a portion of a longer DNA molecule, such as a plasmid or plasmid fragment, that, when read from the 5′ to the 3′ direction, encodes the sequence of amino acids of the specified polypeptide.

All references cited herein are incorporated by reference in their entirety.

The present invention provides dimeric PDGF proteins and materials and methods for making them. The invention is described herein in terms of the representative PDGF-DD dimer. However, those skilled in the art will recognize that the materials and methods of the invention are equally applicable to the production of other PDGF dimers, including, for example, PDGF AA, AB, BB, and CC dimers, and that these dimers, and materials and methods for making them, are within the scope of the invention. Moreover, those skilled in the art will recognize that the invention is not limited to the wild-type sequenes of the PDGF polypeptides, but comprehends the use of variant sequences. PDGF polypeptides and variants thereof, included substituted and truncated variants, are known in the art. See, for example, U.S. Pat. Nos. 4,801,542; 4,845,075; 4,849,407; 4,889,919; and 5,895,755; WIPO Publication WO 00/34474; Betsholtz et al., Nature 320:695-699, 1986; Tong et al., Nature 328:619-621, 1987; and Collins et al., Nature 320:621-624, 1987.

A representative human PDGF-D polypeptide sequence (primary translation product) is shown in SEQ ID NO:2, and a representative mouse PDGF-D polypeptide sequence is shown in SEQ ID NO:4. DNAs encoding these polypeptides are shown in SEQ ID NOS:1 and 3, respectively. Those skilled in the art will recognize that these sequences represent single alleles of the respective human and mouse genes, and that allelic variation is expected to exist. Analysis of the amino acid sequence shown in SEQ ID NO:2 indicates that residues 1 to 18 form a secretory peptide. The primary translation product also includes a CUB domain extending from approximately residue 52 to approximately residue 179; a propeptide-like sequence extending from approximately residue 180 to either residue 245, residue 249 or residue 257 with four potential cleavage sites, including monobasic sites at residue 245 and residue 249, a dibasic site at residues 254-255, and a target site for furin or a furin-like protease at residues 254-257; and the carboxyl-terminal growth factor domain disclosed above. Protein produced by expressing the full-length DNA in a baculovirus expression system showed cleavage between residues 249 and 250, as well as longer species with amino termini at residues 19 and 35. Cleavage of full-length PDGF-DD dimer with plasmin resulted in activation of the protein. By Western analysis, a band migrating at approximately the same size as the growth factor domain was observed. A matched, uncleaved, full-length PDGF-DD sample demonstrated no activity.

While not wishing to be bound by theory, it is believed that the PDGF-D growth factor domain forms anti-parallel dimers, as do the PDGF A and B polypeptides. It is also believed that the two PDGF-D polypeptides are joined by at least one interchain disulfide bond.

The materials and methods of the present invention resulted in enhanced production of PDGF-D growth factor domain dimers. Expression of full-length PDGF-D and the isolated growth factor domain in a baculovirus system resulted in low levels of biologically active protein. Increasing selective pressure did not produce satisfactory expression levels. When a truncated PDGF-D polypeptide beginning at Arg-250 of SEQ ID NO:2 was produced in cultured insect and mammalian cells, a substantial portion of the secreted product was in an inactive, monomeric form. However, the present inventors increased the proportion of biologically active PDGF-DD through the use of a fusion protein. It is believed that this increase was at least in part due to improved dimerization.

Within the present invention, biologically active PDGF-DD proteins comprising two PDGF-D polypeptides are produced by expressing, in a cultured host cell, a polynucleotide encoding a fused polypeptide chain consisting of, from amino to carboxyl terminus, a first PDGF-D growth factor domain polypeptide, a linker polypeptide, and a second PDGF-D growth factor domain polypeptide. Depending upon the type of host cell, the protein is produced as a linear polypeptide lacking disulfide bonds or with the first and second PDGF-D growth factor domain polypeptides joined by at least one disulfide bond. If the protein is produced as a linear polypeptide, the desired disulfide bonds can be formed according to routine methods as disclosed in more detail below.

Within the proteins of the present invention, the PDGF-D growth factor domain polypeptide consists of a sequence of amino acid residues as shown in SEQ ID NO:2 or SEQ ID NO:4 from amino acid x to amino acid y, wherein x is an integer from 246 to 258, inclusive, and y is an integer from 365-370, inclusive. Thus, the PDGF-D growth factor domain polypeptide may consist of, for example, residues 246-370 of SEQ ID NO:2, residues 247-370 of SEQ ID NO:2, residues 248-370 of SEQ ID NO:2, residues 249-370 of SEQ ID NO:2, residues 250-370 of SEQ ID NO:2, residues 251-370 of SEQ ID NO:2, residues 252-370 of SEQ ID NO:2, residues 253-370 of SEQ ID NO:2, residues 254-370 of SEQ ID NO:2, residues 255-370 of SEQ ID NO:2, residues 256-370 of SEQ ID NO:2, residues 257-370 of SEQ ID NO:2, or residues 258-370 of SEQ ID NO:2. Within other embodiments of the invention the PDGF-D growth factor domain polypeptide has an amino-terminus of one of the polypeptides disclosed above, and a carboxyl terminus at residue 365 of SEQ ID NO:2, residue 366 of SEQ ID NO:2, residue 367 of SEQ ID NO:2, residue 368 of SEQ ID NO:2, or residue 369 of SEQ ID NO:2. Within other embodiments the PDGF-D growth factor domain polypeptide consists of the corresponding residues of SEQ ID NO:4.

The PDGF A-chain growth factor domain has an amino terminus at an amino acid residue within residues 1-9, inclusive, of the mature A-chain sequence (U.S. Pat. No. 4,849,407). Within certain embodiments of the invention, the amino termius is within residues 1-5, inclusive. The PDGF A-chain growth factor domain has a carboxyl terminus at an amino acid residue within residues 96-104 of the mature sequence.

The PDGF B-chain growth factor domain has an amino terminus at an amino acid residue within residues 1-15, inclusive, of the mature B-chain sequence (U.S. Pat. No. 4,849,407). Within certain embodiments of the invention, the amino termius is within residues 1-11, inclusive. The PDGF B-chain growth factor domain has a carboxyl terminus at an amino acid residue within residues 102-109 of the mature sequence.

The PDGF C-chain growth factor domain has an amino terminus within residues 226-236, inclusive, of SEQ ID NO:34, and a carboxyl terminus within residues 340-345, inclusive, of SEQ 30 ID NO:34.

In general, the linker polypeptide is designed to provide, upon folding of the fused polypeptide chain, a distance of at least 35 Å between the carboxyl terminus of the first PDGF-D growth factor domain polypeptide and the amino terminus of the second PDGF-D growth factor domain polypeptide. In practice, the linker polypeptide will ordinarily span more than 35 Å to more readily accommodate the three-dimensional structure of the molecule. Hence, linkers spanning 40 Å or more, for example 45 Å or 50 Å, are preferred. Calculation of the effective length of a polypeptide in solution is routine in the art. See, for example, Creighton, Proteins: Structures and Molecular Properties, 2^(nd) edition, W. H. Freeman and Company, 1993, Chapter 5. The linker polypeptide consists of at least 11 amino acid residues and may be as long as 40 residues, commonly not more than 25 residues, and ordinarily not more than 20 residues in length. Thus, the present invention includes, without limitation, the use of linker polypeptides of 12, 13, 14, 15, 16, 17, 18, 19, and 20 residues, with linkers of 14-16 residues particularly preferred.

The linker polypeptide should have an overall hydrophilic character and be non-immunogenic and flexible. As used herein, a “flexible” linker is one that lacks a substantially stable higher-order conformation in solution. Areas of local charge are to be avoided. In general, small, polar, and hydrophilic residues are preferred, and bulky and hydrophobic residues are undesirable. If the linker polypeptide includes charged residues, they will ordinarily be positioned so as to provide a net neutral charge within a small region of the polypeptide. It is therefore preferred to place a charged residue adjacent to a residue of opposite charge. In general, preferred residues for inclusion within the linker polypeptide include Gly, Ser, Ala, Thr, Asn, and Gln; more preferred residues include Gly, Ser, Ala, and Thr; and the most preferred residues are Gly and Ser. In general, Phe, Tyr, Trp, Cys, Pro, Leu, Ile, Lys, and Arg residues will be avoided, Cys residues due to their potential for formation of unwanted disulfide bonds, Pro residues due to their hydrophobicity and lack of flexibility, and Lys and Arg residues due to potential immunogenicity. However, these less desirable residues may be included to provide a specific proteolytic cleavage site as disclosed below. Within certain embodiments of the invention the linker polypeptide comprises a proteolytic cleavage site to facilitate removal of the peptide link between the first and second PDGF-D growth factor domain polypeptides. Exemplary proteolytic cleavage sites include sequences cleaved by plasmin, thrombin, factor Xa, enterokinase, furin, and rhinovirus 3C protease. The use of these and other proteases to cleave fusion proteins is known in the art. See, for example, Rubinstein et al., WO 00/61768; van de Ven et al., U.S. Pat. No. 5,935,815; and Fischer et al., U.S. Pat. No. 6,010,844. Thrombin cleaves after the dipeptide sequence Arg-Pro or Pro-Arg. Enterokinase cleaves after the pentapeptide sequence Asp-Asp-Asp-Asp-Lys (SEQ ID NO:5). Factor Xa cleaves after the sequence Ile-Glu-Gly-Arg (SEQ ID NO:6). Plasmin cleaves after the sequence Arg-Pro. The human rhinovirus 3C protease cleaves Gln-Gly peptide bonds, such as in the sequence Leu-Glu-Val-Leu-Phe-Gln-Gly-Pro (SEQ ID NO:7). Furin cleaves after Arg-Xaa-Lys/Arg-Arg (SEQ ID NO:8). Exemplary linkers are those having two repeats of the structure Ser-Gly-Ser-Gly-Ser (SEQ ID NO:9) in combination with a proteolytic cleavage site, such as the linker Ser-Gly-Ser-Gly-Ser-Ser-Gly-Ser-Gly-Ser-Leu-Val-Pro-Arg-Gly-Ser (SEQ ID NO:10), which includes a thrombin cleavage site.

The present invention also provides polynucleotide molecules, including DNA and RNA molecules, that encode the PDGF-D polypeptides disclosed above. The polynucleotides of the present invention include both single-stranded and double-stranded molecules. A representative DNA sequence encoding human PDGF-D is set forth in SEQ ID NO:1, and a representative DNA sequence encoding mouse PDGF-D is set forth in SEQ ID NO:3. Additional DNA sequences encoding PDGF-D polypeptides can be readily generated by those of ordinary skill in the art based on the genetic code. Counterpart RNA sequences can be generated by substitution of U for T. Those skilled in the art will readily recognize that, in view of the degeneracy of the genetic code, considerable sequence variation is possible among polynucleotide molecules encoding PDGF-D polypeptides.

Methods for preparing DNA and RNA are well known in the art. Complementary DNA (cDNA) clones are prepared from RNA that is isolated from a tissue or cell that produces large amounts of PDGF-D RNA. Such tissues and cells are identified by Northern blotting (Thomas, Proc. Natl. Acad. Sci. USA 77:5201, 1980), and include heart, pancreas, stomach, and adrenal gland. Total RNA can be prepared using guanidine HCl extraction followed by isolation by centrifugation in a CsCl gradient (Chirgwin et al., Biochemistry 18:52-94, 1979). Poly (A)⁺ RNA is prepared from total RNA using the method of Aviv and Leder (Proc. Natl. Acad. Sci. USA 69:1408-1412, 1972). Complementary DNA is prepared from poly(A)⁺ RNA using known methods. In the alternative, genomic DNA can be isolated. For some applications (e.g., expression in transgenic animals) it may be advantageous to use a genomic clone, or to modify a cDNA clone to include at least one genomic intron. Methods for identifying and isolating cDNA and genomic clones are well known and within the level of ordinary skill in the art, and include the use of the sequences disclosed herein, or parts thereof, for probing or priming a library. Polynucleotides encoding PDGF-D polypeptides are identified and isolated by, for example, hybridization or polymerase chain reaction (“PCR”, Mullis, U.S. Pat. No. 4,683,202). Expression libraries can be probed with antibodies to PDGF-D, receptor fragments, or other specific binding partners.

The polynucleotides of the present invention can also be prepared by automated synthesis. The production of short, double-stranded segments (60 to 80 bp) is technically straightforward and can be accomplished by synthesizing the complementary strands and then annealing them. Longer segments (typically >300 bp) are assembled in modular form from single-stranded fragments that are from 20 to 100 nucleotides in length. Automated synthesis of polynucleotides is within the level of ordinary skill in the art, and suitable equipment and reagents are available from commercial suppliers. See, in general, Glick and Pasternak, Molecular Biotechnology, Principles & Applications of Recombinant DNA, ASM Press, Washington, D.C., 1994; Itakura et al., Ann. Rev. Biochem. 53: 323-356, 1984; and Climie et al., Proc. Natl. Acad. Sci. USA 87:633-637, 1990.

The proteins of the present invention can be produced in genetically engineered host cells according to conventional techniques. Suitable host cells are those cell types that can be transformed or transfected with exogenous DNA and grown in culture, and include bacteria, fungal cells, and cultured higher eukaryotic cells (including cultured cells of multicellular organisms). Techniques for manipulating cloned DNA molecules and introducing exogenous DNA into a variety of host cells are disclosed by Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989, and Ausubel et al., eds., Current Protocols in Molecular Biology, Green and Wiley and Sons, NY, 1993.

In general, a DNA sequence encoding a PDGF-D polypeptide is operably linked to other genetic elements required for its expression, generally including a transcription promoter and terminator, within an expression vector. The vector will also commonly contain one or more selectable markers and one or more origins of replication, although those skilled in the art will recognize that within certain systems selectable markers may be provided on separate vectors, and replication of the exogenous DNA may be provided by integration into the host cell genome. Selection of promoters, terminators, selectable markers, vectors, and other elements is a matter of routine design within the level of ordinary skill in the art. Many such elements are described in the literature and are available through commercial suppliers.

To direct a PDGF-D polypeptide into the secretory pathway of a host cell, a secretory signal sequence (also known as a leader sequence, prepro sequence or pre sequence) is provided in the expression vector. The secretory signal sequence may be that of a PDGF-D, or may be derived from another secreted protein (e.g., t-PA; see, U.S. Pat. No. 5,641,655) or synthesized de novo. The secretory signal sequence is operably linked to the PDGF-D DNA sequence, i.e., the two sequences are joined in the correct reading frame and positioned to direct the newly synthesized polypeptide into the secretory pathway of the host cell. Secretory signal sequences are commonly positioned 5′ to the DNA sequence encoding the PDGF-D polypeptide, although certain signal sequences may be positioned elsewhere in the DNA sequence of interest (see, e.g., Welch et al., U.S. Pat. No. 5,037,743; Holland et al., U.S. Pat. No. 5,143,830).

Expression of PDGF-D polypeptides (including the fusion proteins of the present invention) via a host cell secretory pathway is expected to result in the production of dimeric proteins. Dimers may also be assembled in vitro upon incubation of PDGF-D polypeptides under suitable conditions. In general, in vitro assembly will include incubating the polypeptides under denaturing and reducing conditions followed by refolding and reoxidation of the polypeptides to form dimers. Recovery and assembly of proteins expressed in bacterial cells is disclosed below.

Cultured mammalian cells are suitable hosts for use within the present invention. Methods for introducing exogenous DNA into mammalian host cells include calcium phosphate-mediated transfection (Wigler et al., Cell 14:725, 1978; Corsaro and Pearson, Somatic Cell Genetics 7:603, 1981: Graham and Van der Eb, Virology 52:456, 1973), electroporation (Neumann et al., EMBO J. 1:841-845, 1982), DEAE-dextran mediated transfection (Ausubel et al., ibid.), and liposome-mediated transfection (Hawley-Nelson et al., Focus 15:73, 1993; Ciccarone et al., Focus 15:80, 1993). The production of recombinant polypeptides in cultured mammalian cells is disclosed by, for example, Levinson et al., U.S. Pat. No. 4,713,339; Hagen et al., U.S. Pat. No. 4,784,950; Palmiter et al, U.S. Pat. No. 4,579,821; and Ringold, U.S. Pat. No. 4,656,134. Suitable cultured mammalian cells include the COS-1 (ATCC No. CRL 1650), COS-7 (ATCC No. CRL 1651), BHK (ATCC No. CRL 1632), BHK 570 (ATCC No. CRL 10314), 293 (ATCC No. CRL 1573; Graham et al., J. Gen. Virol. 36:59-72, 1977) and Chinese hamster ovary (e.g. CHO-K1; ATCC No. CCL 61) cell lines. Additional suitable cell lines are known in the art and available from public depositories such as the American Type Culture Collection, Manassas, Va. Strong transcription promoters can be used, such as promoters from SV-40 or cytomegalovirus. See, e.g., U.S. Pat. No. 4,956,288. Other suitable promoters include those from metallothionein genes (U.S. Pat. Nos. 4,579,821 and 4,601,978) and the adenovirus major late promoter. Expression vectors for use in mammalian cells include pZP-1 and pZP-9, which have been deposited with the American Type Culture Collection, 10801 University Blvd., Manassas, Va. USA under accession numbers 98669 and 98668, respectively, and derivatives of these vectors.

Drug selection is generally used to select for cultured mammalian cells into which foreign DNA has been inserted. Such cells are commonly referred to as “transfectants”. Cells that have been cultured in the presence of the selective agent and are able to pass the gene of interest to their progeny are referred to as “stable transfectants.” An exemplary selectable marker is a gene encoding resistance to the antibiotic neomycin. Selection is carried out in the presence of a neomycin-type drug, such as G-418 or the like. Selection systems can also be used to increase the expression level of the gene of interest, a process referred to as “amplification.” Amplification is carried out by culturing transfectants in the presence of a low level of the selective agent and then increasing the amount of selective agent to select for cells that produce high levels of the products of the introduced genes. An exemplary amplifiable selectable marker is dihydrofolate reductase, which confers resistance to methotrexate. Other drug resistance genes (e.g. hygromycin resistance, multi-drug resistance, puromycin acetyltransferase) can also be used.

Other higher eukaryotic cells can also be used as hosts, including insect cells, plant cells and avian cells. The use of Agrobacterium rhizogenes as a vector for expressing genes in plant cells has been reviewed by Sinkar et al., J. Biosci. (Bangalore) 11:47-58, 1987. Transformation of insect cells and production of foreign polypeptides therein is disclosed by Guarino et al., U.S. Pat. No. 5,162,222 and WIPO publication WO 94/06463.

Insect cells can be infected with recombinant baculovirus, commonly derived from Autographa californica nuclear polyhedrosis virus (AcNPV). See, King and Possee, The Baculovirus Expression System: A Laboratory Guide, London, Chapman & Hall; O'Reilly et al., Baculovirus Expression Vectors: A Laboratory Manual, New York, Oxford University Press., 1994; and Richardson, Ed., Baculovirus Expression Protocols. Methods in Molecular Biology, Humana Press, Totowa, N.J., 1995. Recombinant baculovirus can also be produced through the use of a transposon-based system described by Luckow et al. (J. Virol. 67:4566-4579, 1993). This system, which utilizes transfer vectors, is commercially available in kit form (BAC-TO-BAC kit; Life Technologies, Gaithersburg, Md.). The transfer vector (e.g., PFASTBAC1; Life Technologies) contains a Tn7 transposon to move the DNA encoding the protein of interest into a baculovirus genome maintained in E. coli as a large plasmid called a “bacmid.” See, Hill-Perkins and Possee, J. Gen. Virol. 71:971-976, 1990; Bonning et al., J. Gen. Virol. 75:1551-1556, 1994; and Chazenbalk and Rapoport, J. Biol. Chem. 270:1543-1549, 1995. In addition, transfer vectors can include an in-frame fusion with DNA encoding a polypeptide extension or affinity tag as disclosed above. Using techniques known in the art, a transfer vector containing a PDGF-D polypeptide-encoding sequence is transformed into E. coli host cells, and the cells are screened for bacmids which contain an interrupted lacZ gene indicative of recombinant baculovirus. The bacmid DNA containing the recombinant baculovirus genome is isolated, using common techniques, and used to transfect Spodoptera frugiperda cells, such as Sf9 cells. Recombinant virus that expresses PDGF-D protein is subsequently produced. Recombinant viral stocks are made by methods commonly used the art.

For protein production, the recombinant virus is used to infect host cells, typically a cell line derived from the fall armyworm, Spodoptera frugiperda (e.g., Sf9 or Sf21 cells) or Trichoplusia ni (e.g., HIGH FIVE cells; Invitrogen, Carlsbad, Calif.). See, in general, Glick and Pastemak, Molecular Biotechnology: Principles and Applications of Recombinant DNA, ASM Press, Washington, D.C., 1994. See also, U.S. Pat. No. 5,300,435. Serum-free media are used to grow and maintain the cells. Suitable media formulations are known in the art and can be obtained from commercial suppliers. The cells are grown up from an inoculation density of approximately 2-5×10⁵ cells to a density of 1-2×10⁶ cells, at which time a recombinant viral stock is added at a multiplicity of infection (MOI) of 0.1 to 10, more typically near 3. Procedures used are generally described in available laboratory manuals (e.g., King and Possee, ibid.; O'Reilly et al., ibid.; Richardson, ibid.).

Fungal cells, including yeast cells, can also be used within the present invention. Yeast species of particular interest in this regard include Saccharomyces cerevisiae, Pichia pastoris, and Pichia methanolica. Methods for transforming S. cerevisiae cells with exogenous DNA and producing recombinant polypeptides therefrom are disclosed by, for example, Kawasaki, U.S. Pat. No. 4,599,311; Kawasaki et al., U.S. Pat. No. 4,931,373; Brake, U.S. Pat. No. 4,870,008; Welch et al., U.S. Pat. No. 5,037,743; and Murray et al., U.S. Pat. No. 4,845,075. Transformed cells are selected by phenotype determined by the selectable marker, commonly drug resistance or the ability to grow in the absence of a particular nutrient (e.g., leucine). An exemplary vector system for use in Saccharomyces cerevisiae is the POT1 vector system disclosed by Kawasaki et al. (U.S. Pat. No. 4,931,373), which allows transformed cells to be selected by growth in glucose-containing media. Suitable promoters and terminators for use in yeast include those from glycolytic enzyme genes (see, e.g., Kawasaki, U.S. Pat. No. 4,599,311; Kingsman et al., U.S. Pat. No. 4,615,974; and Bitter, U.S. Pat. No. 4,977,092) and alcohol dehydrogenase genes. See also U.S. Pat. Nos. 4,990,446; 5,063,154; 5,139,936 and 4,661,454. Transformation systems for other yeasts, including Hansenula polymorpha, Schizosaccharomyces pombe, Kluyveromyces lactis, Kluyveromyces fragilis, Ustilago maydis, Pichia pastoris, Pichia methanolica, Pichia guillermondii and Candida maltosa are known in the art. See, for example, Gleeson et al., J. Gen. Microbiol. 132:3459-3465, 1986; Cregg, U.S. Pat. No. 4,882,279; and Raymond et al., Yeast 14:11-23, 1998. Aspergillus cells may be utilized according to the methods of McKnight et al., U.S. Pat. No. 4,935,349. Methods for transforming Acremonium chrysogenum are disclosed by Sumino et al., U.S. Pat. No. 5,162,228. Methods for transforming Neurospora are disclosed by Lambowitz, U.S. Pat. No. 4,486,533. Production of recombinant proteins in Pichia methanolica is disclosed in U.S. Pat. Nos. 5,716,808; 5,736,383; 5,854,039; and 5,888,768.

Prokaryotic host cells, including strains of the bacteria Escherichia coli, Bacillus and other genera are also useful host cells within the present invention. Techniques for transforming these hosts and expressing foreign DNA sequences cloned therein are well known in the art (see, e.g., Sambrook et al., ibid.). When expressing a zvegf4 polypeptide in bacteria such as E. coli, the polypeptide may be retained in the cytoplasm, typically as insoluble granules, or may be directed to the periplasmic space by a bacterial secretion sequence. In the former case, the cells are lysed, and the granules are recovered and denatured using, for example, guanidine isothiocyanate or urea. The denatured polypeptide can then be refolded and dimerized by diluting the denaturant, such as by dialysis against a solution of urea and a combination of reduced and oxidized glutathione, followed by dialysis against a buffered saline solution. In the alternative, the protein may be recovered from the cytoplasm in soluble form and isolated without the use of denaturants. The protein is recovered from the cell as an aqueous extract in, for example, phosphate buffered saline. To capture the protein of interest, the extract is applied directly to a chromatographic medium, such as an immobilized antibody or heparin-Sepharose column. Secreted polypeptides can be recovered from the periplasmic space in a soluble and functional form by disrupting the cells (by, for example, sonication or osmotic shock) to release the contents of the periplasmic space and recovering the protein, thereby obviating the need for denaturation and refolding.

Transformed or transfected host cells are cultured according to conventional procedures in a culture medium containing nutrients and other components required for the growth of the chosen host cells. A variety of suitable media, including defined media and complex media, are known in the art and generally include a carbon source, a nitrogen source, essential amino acids, vitamins and minerals. Media may also contain such components as growth factors or serum, as required. The growth medium will generally select for cells containing the exogenously added DNA by, for example, drug selection or deficiency in an essential nutrient which is complemented by the selectable marker carried on the expression vector or co-transfected into the host cell.

When the linker polypeptide comprises a proteolytic cleavage site, the PDGF-DD polypeptide can be cleaved within the host cell if the host cell produces a protease that cleaves at the cleavage site. If the host cell does not naturally produce the protease, it can be transfected to co-express the protease and the PDGF-DD polypeptide. See, for example, U.S. Pat. Nos. 5,648,254 and 5,935,815. Such intracellular cleavage of the PDGF-DD polypeptide will facilitate the secretion of a disulfide-bonded, dimeric protein.

The PDGF-DD proteins of the present invention that contain a cleavage site in the linker polypeptide can also be cleaved in vitro according to conventional methods. The use of proteases for processing recombinant proteins is routine in the art and includes the use of immobilized proteases. See, for example, U.S. Pat. No. 6,010,844. Specific reaction conditions are based on the protease to be used and will be adjusted to minimize unwanted proteolysis with the first polypeptide segment. In general, such parameters as reaction time and ratio of protease to substrate will be adjusted to obtain the desired result.

PDGF-DD proteins of the present invention are purified by conventional protein purification methods, typically by a combination of chromatographic techniques. See, in general, Affinity Chromatography: Principles & Methods, Pharmacia LKB Biotechnology, Uppsala, Sweden, 1988; and Scopes, Protein Purification: Principles and Practice, Springer-Verlag, New York, 1994. Suitable chromatographic techniques include, without limitation, cation-exchange chromatography, hydrophobic interaction chromatography, size-exclusion chromatography, and affinity chromatography.

PDGF-DD proteins can be used wherever it is desired to stimulate the production of bone and/or connective tissue in both humans and non-human animals. Veterinary uses include use in domestic animals, including livestock and companion animals. Specific applications include, without limitation, fractures, including non-union fractures and fractures in patients with compromised healing, such as diabetics, alcoholics, and the aged; bone grafts; healing bone following radiation-induced osteonecrosis; implants, including joint replacements and dental implants; repair of bony defects arising from surgery, such as cranio-maxilofacial repair following tumor removal, surgical reconstruction following tramatic injury, repair of hereditary or other physical abnormalities, and promotion of bone healing in plastic surgery; treatment of periodontal disease and repair of other dental defects; treatment of bone defects following therapeutic treatment of bone cancers; increase in bone formation during distraction osteogenesis; treatment of joint injuries, including repair of cartilage and ligament; repair of joints that have been afflicted with osteoarthritis; tendon repair and re-attachment; treatment of osteoporosis (including age-related osteoporosis, post-menopausal osteoporosis, glutocorticoid-induced osteoporosis, and disuse osteoporosis) and other conditions characterized by increased bone loss or decreased bone formation; elevation of peak bone mass in pre-menopausal women; and use in the healing of connective tissues associated with dura mater.

For pharmaceutical use, PDGF-DD proteins are formulated for local or systemic (particularly intravenous or subcutaneous) delivery according to conventional methods. In general, pharmaceutical formulations will include a PDGF-DD protein in combination with a pharmaceutically acceptable delivery vehicle. Delivery vehicles include biocompatible solid or semi-solid matrices, including powdered bone, ceramics, biodegradable and non-biodegradable synthetic polymers, and natural polymers; tissue adhesives (e.g., fibrin-based); aqueous polymeric gels; aqueous solutions; liposomes; ointments; and the like. These and other suitable vehicles are known in the art. Formulations may further include one or more additional growth factors, excipients, preservatives, solubilizers, buffering agents, albumin to prevent protein loss on vial surfaces, etc. Methods of formulation are well known in the art and are disclosed, for example, in Remington: The Science and Practice of Pharmacy, 20th ed., Gennaro et al., eds., Lippincott, Williams & Wilkins, Baltimore, 2000. The exact dose will be determined by the clinician according to accepted standards, taking into account the nature and severity of the condition to be treated, patient traits, etc. Determination of dose is within the level of ordinary skill in the art. Depending upon the route and method of administration, the protein may be administered in a single dose, as a prolonged infusion, or intermittently over an extended period. Intravenous administration will be by bolus injection or infusion over a typical period of one to several hours. Sustained release formulations can also be employed. In general, a therapeutically effective amount of zvegf4 is an amount sufficient to produce a clinically significant change in the treated condition, such as a clinically significant reduction in time required for fracture repair, a significant reduction in the volume of a void or other defect, a significant increase in bone density, a significant reduction in morbidity, or a significantly increased histological score.

PDGF-DD will ordinarily be used in a concentration of about 10 to 100 μg/ml of total volume, although concentrations in the range of 1 ng/ml to 1000 μg/ml may be used. For local application, such as for the regeneration of bone in a fracture or other bony defect, the protein will be applied in the range of 0.1-100 μg/cm² of wound area.

PDGF-DD proteins of the present invention can be used in combination with other growth factors and other therapeutic agents that have a positive effect on the growth of bone or connective tissue. Such growth factors include insulin-like growth factor 1 (IGF-1), PDGF, alpha and beta transforming growth factors (TGF-α and TGF-β), epidermal growth factor (EGF), bone morphogenetic proteins, leukemia inhibitory factor, and fibroblast growth factors. Other therapeutic agents include vitamin D, bisphosphonates, calcitonin, estrogens, parathyroid hormone, osteogenin, NaF, osteoprotegerin, and statins.

The invention is further illustrated by the following, non-limiting examples.

EXAMPLE 1

An expression vector, zVegf4[GFD]-FL2-TCS-zVegf4[GFD]/pZBV37L, was prepared to express PDGF-DD protein in insect cells. The vector was designed to express two PDGF-D growth factor domain polypeptides (zvegf4 GFD) connected by a ten amino acid Ser-Gly-rich sequence and a six amino acid thrombin cleavage site (TCS) (SEQ ID NO:10). The protein was designated “zvegf4-sc-GFD.”

An 830-bp fragment (designated zvegf4[GFD]-FL2-TCS-zvegf4[GFD]), containing BspEI and EcoRI restriction sites on the 5′ and 3′ ends, respectively, was generated by ligating two PCR-amplified fragments. The first fragment contained BspEI and BamHI sites on the 5′ and 3′ ends, respectively, and the second fragment contained BamHI and EcoRI sites on the 5′ and 3′ ends, respectively. The first fragment was generated by PCR amplification from a plasmid containing PDGF-D cDNA using primers zc38,955 (SEQ ID NO:11) and zc38,602 (SEQ ID NO:12). The second fragment was generated by PCR amplification from a plasmid containing PDGF-D cDNA using primers zc38,950 (SEQ ID NO:13) and zc38,953 (SEQ ID NO:14). The PCR reaction mixtures were incubated at 94° C. for 5 minutes followed by 35 cycles of 94° C. for 60 seconds, 58° C. for 120 seconds, and 72° C. for 180 seconds; held at at 72° C. for 10 minutes; followed by a 4° C. soak. The first fragment was digested with BspEI and BamHI, and the second fragment was digested with BamHI and EcoRI. The fragments were run on an agarose gel, excised, and purified using a spin column containing a silica gel membrane (QIAQUICK Gel Extraction Kit; Qiagen, Inc.). The resulting purified first and second fragments were ligated and electrophoresed on an agarose gel. A band corresponding to the combined sizes of the fragments was excised and purified using a spin column. The purified fragment was ligated into the vector pZBV37L, a modification of the PFASTBAC1 expression vector (Life Technologies, Gaithersburg, Md.) in which the polyhedron promoter had been removed and replaced with the late activating Basic Protein Promoter. Approximately 23 ng of the restriction digested zVegf4[GFD]-FL2-TCS-zVegf4[GFD] insert and about 50 ng of the pZBV37L vector were ligated overnight at 16° C. The ligation mixture was transformed into E. coli host cells (ELECTROMAX DH12S; Life Technologies) by electroporation at 400 Ohms, 2V, and 25 μF in a 2-mm gap electroporation cuvette (BTX, Model No. 620). The transformed cells were diluted in 450 μl of SOC media (2% BACTO Tryptone (Difco Laboratories, Detroit, Mich.), 0.5% BACTO Yeast Extract (Difco Laboratories), 0.01 M NaCl, 1.5 mM KC1, 10 mM MgCl₂, 10 mM MgSO₄, and 20 mM glucose), and 100 μl of the dilution was plated onto LB plates containing 100 μg/ml ampicillin. Clones were analyzed by PCR, and three positive clones were selected to be outgrown and purified using a spin column (QIAPREP Spin Miniprep Kit; Qiagen, Inc.). The positive clones were also confirmed by sequencing. Two μl of each of the positive clones was transformed into 20 μl E. coli cells (MAX EFFICIENCY DH10BAC Competent Cells; Life Technologies) by heat shock for 45 seconds in a 42° C. heat block. The transformed cells were diluted in 980 μl SOC media, and 100 μl was plated onto Luria Agar plates containing 50 μg/ml kanamycin, 7 μg/ml gentamicin, 10 μg/ml tetracycline, 40 μg/ml IPTG and 200 μg/ml halogenated indolyl-β-D-galactoside (bluo-gal). The plates were incubated for 48 hours at 37° C. A color selection was used to identify those cells having transposed viral DNA (referred to as a “bacmid”). Colonies that were white in color were picked for analysis and analyzed by PCR, and positive colonies (containing the desired bacmid) were selected for outgrowth and purified using a spin column. Clones were screened for the correct insert by amplifying DNA by PCR using primers to the transposable element in the bacmid (primers zc447 (SEQ ID NO:15) and zc976 (SEQ ID NO:16)). The PCR mixture was incubated at 94° C. for 5 minutes; 30 cycles of 94° C. for 60 seconds, 50° C. for 120 seconds, and 72° C. for 180 seconds; 72° C. for 10 min; followed by a 4° C. soak. The PCR product was run on a 1% agarose gel to check the insert size.

Clones having the correct insert size were used to transfect Spodoptera frugiperda (Sf9) cells. Sf9 cells were seeded at 1×10⁶ cells per well in a 6-well plate and allowed to attach for 1 hour at 27° C. Ten microliters of bacmid DNA was diluted with 100 μl Sf-900 II SFM (Life Technologies). Twenty μl of a 3:1 (w/w) liposome formulation of the polycationic lipid 2,3-dioleyloxy-N-[2(sperminecarboxamido)ethyl]-N,N-dimethyl-1-propaniminium-trifluoroacetate and the neutral lipid dioleoyl phosphatidylethanolamine in membrane-filtered water (LIPOFECTAMINE™ Reagent; Life Technologies) was diluted with 100 μl Sf-900 II SFM. The bacmid DNA and lipid solutions were gently mixed and incubated 30-45 minutes at room temperature. The media from one well of cells was aspirated, and the cells were washed once with 2 ml fresh Sf-900 II SFM media. Eight hundred microliters of Sf-900 II SFM was added to the lipid-DNA mixture. The wash media was aspirated, and the DNA-lipid mixture was added to the cells. The cells were incubated at 27° C. overnight. The DNA-lipid mixture was aspirated, and 2 ml of Sf-900 II media was added to each plate. The plates were incubated at 27° C., 90% humidity, for 96 hours, after which the virus was harvested.

For the primary viral amplification, Sf9 cells were grown in 50 ml Sf-900 II SFM in a 125-ml shake flask to an approximate density of 1×10⁶ cells/ml. They were then infected with 500 μl of the viral stock from the transfected 6-well plate and incubated at 27° C. for 3 days, after which the virus was harvested and titered.

EXAMPLE 2

A 50-ml culture of Sf9 cells at 2×10⁶ cells/ml in a 125-ml shake flask at 27° C. was infected with 5 ml of primary amplified virus encoding human zvegf4-sc-GFD. The culture was harvested at 48 hours. The supernatant and a whole cell lysate were analyzed by western blotting. A band at 36 kDa, approximately the correct size, was seen in lanes corresponding to both supernatant and lysate samples.

Viral stocks were amplified in large scale. 1 L shake flasks at 27° C. with 1×10⁶ cells/ml were infected with the virus at a target MOI of 0.1 and incubated for 96 hours, at which time the supernatant was harvested and titered.

Large scale amplified virus was used to infect multiple 1-liter cultures of Sf9 cells at 2×10⁶ cells/ml at an MOI of approximately 1 to 2. Supernatant was harvested at 48 hours.

EXAMPLE 3

Recombinant zvegf4-sc-GFD protein was recovered from conditioned culture media of baculovirus-infected insect cells. Ten 1-liter cultures were harvested, and the media were filtered using a 0.20 μm filter.

Protein was partially purified from the conditioned media by a combination of cation exchange, hydrophobic interaction, and size-exclusion chromatographies. Filtered culture medium (pH 6.3, conductivity 9 mS) was directly loaded onto a 50 ml cation-exchange column (POROS 50-HS; Applied Biosystems, Framingham, Mass.). The column was washed with ten column volumes (cv) of 96% starting buffer A (50 mM 2-(N-morpholino) ethanesulfonic acid (MES), pH 6.5) and 4% of Buffer B (50 mM MES, 2M NaCl, pH 6.6), and the bound protein was eluted with a 15-cv linear gradient to 100% buffer B, followed by 5 cv of 100% buffer B at 12 ml/min. Four-ml fractions were collected. Samples from the column were analyzed by SDS-PAGE with silver staining and western blotting for the presence of zvegf4-sc-GFD protein. Protein-containing fractions were pooled, mixed with ammonium sulfate pellet to a final concentration of 2 M, filtered through a 0.20 μm filtration unit (NALGENE), and loaded onto an 20-ml derivatized agarose column (Phenyl SEPHAROSE; Amersham Pharmacia Biotech). Proteins were eluted with a 3-cv linear gradient from 100% buffer C (50 mM sodium phosphate, 2 M ammonium sulfate, pH 7.0) to 40%, followed by a 10-cv linear gradient from 40% buffer C to 50 mM sodium phosphate, pH 7.0. Twelve-ml fractions were collected. The western blotting positive signal fractions were pooled and concentrated to 1 ml using Millipore concentrator-80 (Millipore) and loaded onto a 16×450 mm cross-linked dextran gel filtration column (SUPERDEX-75; Amersham Pharmacia Biotech) at 1 ml/min. Every two fractions containing purified zvegf4-sc-GFD were pooled and concentrated for assay.

Recombinant zvegf4-sc-GFD protein was analyzed by SDS-PAGE (Bis-Tris NUPAGE gel, 4-12%; Invitrogen, Carlsbad, Calif.) with silver staining (Fast Silver; Geno Tech, St. Louis, Mo.) and Western blotting using a monoclonal antibody to PDGF-D protein. Either the conditioned media or purified protein was electrophoresed using a commercially available blotting apparatus (NOVEX XCELL II mini-cell; Invitrogen) and transferred to nitrocellulose filters (0.2 μm; Invitrogen) at room temperature using a blot module (XCELL II; Invitrogen) with stirring according to directions provided in the instrument manual. The transfer was run at 500 mA for one hour in a buffer containing 25 mM Tris base, 200 mM glycine, and 20% methanol. The blots were then blocked with 10% non-fat dry milk in PBS for 10 minutes at room temperature. The blots were quickly rinsed, then the mouse primary antibody, diluted 1:2000 in PBS containing 2.5% non-fat dry milk, was added. The blots were incubated for two hours at room temperature or overnight at 4° C. with gentle shaking. Following the incubation, blots were washed three times for 10 minutes each in PBS, then labeled with a secondary antibody (goat anti-mouse IgG conjugated to horseradish peroxidase; Pierce Chemical Co., Rockford, Ill.) diluted 1:2000 in PBS containing 2.5% non-fat dry milk, and the blots were incubated for two hours at room temperature with gentle shaking. The blots were then washed three times, 10 minutes each, in PBS, then quickly rinsed with H₂O. The blots were developed using commercially available chemiluminescent substrate reagents (SUPERSIGNAL ULTRA reagents 1 and 2 mixed 1:1; reagents obtained from Pierce Chemical Co., Rockford, Ill.), and the signal was captured using commercially available software (LUMI-IMAGER LumiAnalyst 3.0; Boehringer Mannheim GmbH, Germany) for times ranging from 10 seconds to 5 minutes or as necessary.

The partially purified zvegf4-sc-GFD protein appeared as two bands on the silver-stained gel at about 32 and 20 kDa under both non-reducing conditions and reducing conditions. This result suggested existence of a two-tandem repeat form of zvegf4-sc-GFD and a zvegf4-GFD monomer possibly cleaved around the linker region of two zvegf4-GFD repeats. The partially purified protein was very active in a cell-based luciferase assay.

EXAMPLE 4

Rat stellate cells were grown in 96-well tissue clusters (FALCON; BD, Franklin Lakes, N.J.) in DMEM (Life Technologies, Gaithersburg, Md.) supplemented with 10% fetal bovine serum (Hyclone Laboratories, Inc., Logan, Utah). The next day, the medium was switched to serum-free medium by substituting 0.1% BSA (Fraction V, Sigma, St. Louis, Mo.) for serum. This medium also contained the adenoviral construct KZ136 that encodes a luciferase reporter mini-gene driven by SRE and STAT elements, at a 1000:1 multiplicity of infection (m.o.i.). After allowing 24 hours for the incorporation of the adenoviral construct into the cells, the media were changed and replaced with serum-free media +0.1% BSA that contained conditioned media (CM) from insect cells expressing zvegf4-sc-GFD or control cells at the indicated final concentration (Table, below). Four hours later the cells were lysed, and luciferase activity, denoting activation of the reporter gene, was determined in the lysate using a commercially available assay kit (obtained from Promega Corp., Madison, Wis.) and a luminescence reader (MICROLUMAT PLUS, Berthold Technologies, Bad Wildbad, Germany). Results (shown in the Table) were obtained as relative luciferase units (RLU) in the lysate, with a value of 1 assigned to the BSA control (no conditioned medium added to the cells). TABLE Control CM zvegf4-sc-GFD CM BSA (Basal) 1 1 CM 1:30 dilution 1 2 CM 1:10 dilution 1 2.6 CM 1:3 dilution 1 3.7

Conditioned media from insect cells that were transfected with the baculovirus expressing zvegf4-sc-GFD contained a protein that reacted with anti-PDGF-D antibodies and migrated at the right position on an SDS-protein gel (about 35 kDa non-reduced). By this method, protein concentration was estimated to be approximately 0.4 μg/ml. Conditioned media from untrasfected cells did not contain any moieties recognized by anti-PDGF-D antibodies.

Based on a standard curve using purified, recombinant PDGF-D-GFD protein (produced by expressing a DNA encoding the growth factor domain alone), the activity of the zvegf4-sc-GFD CM was roughly equivalent to 0.3 μg/ml of PDGF-D-GFD. This correlated with the concentration of this protein in the CM as determined by western blotting. It also showed that the single-chain GFD dimer was biologically active and that its activity weight per weight was comparable to that of PDGF-D-GFD.

EXAMPLE 5

A polynucleotide sequence of human zvegf4-sc-GFD was prepared. Two zvegf4 cDNA fragments were isolated by PCR from the template zVegf4[GFD]-FL2-TCS-zVegf4[GFD]/pZBV37L (Example 1). For the first fragment, two primers were used to synthesize the first half of the single chain molecule: (1) primer zc40,105 (SEQ ID NO:17), containing 41 bp of the vector flanking sequence and 25 bp corresponding to the amino terminus of the zvegf4 sequence; and (2) primer zc40,369 (SEQ ID NO: 18), containing 100% homology internal to the zvegf4 template, of which 30 bp contained homology to fragment 2. The second fragment was synthesized using primer zc40,368 (SEQ ID NO:19), which contained 100% homology to the zvegf4 template, of which 30 bp contained homology to fragment 1, and primer zc40,106 (SEQ ID NO:20), which contained 40 bp of the vector flanking sequence and 25 bp corresponding to the carboxyl terminus of the zvegf4 sequence. The PCR reactions were run for 25 cycles of 94° C. for 30 seconds, 55° C. for 30 seconds, and 72° C. for 1 minute, followed by a 4° C. soak. Two μl of each of the 100-μl PCR reaction mixtures was run on a 1.0% agarose gel with 1× TBE buffer for analysis, and the expected bands of approximately 400 bp were seen. The remaining 98 μl of each PCR reaction mixture was precipitated with 200 μl of absolute ethanol for use in constructing an expression vector encoding the zvegf4-sc-GFD (pTAP339).

pTAP168 was generated by inserting a PCR-generated linker fragment into the SmaI site of pCZR239 via homologous recombination in yeast (Saccharomyces cerevisiae). (Plasmid pCZR239 was derived from the plasmids pRS316 (a Saccharomyces cerevisiae shuttle vector; Hieter and Sikorski, Genetics 122:19-27, 1989) and pMAL-c2 (obtained from New England Biolabs, Beverly, Mass.), an E. coli expression plasmid comprising the tac promoter driving MalE (encoding maltose binding protein) followed by a His tag, a thrombin cleavage site, a cloning site, and the rrnB terminator. Plasmid pCZR239 contains a kanamycin resistance gene in which the SmaI site has been destroyed, and has NotI and SfiI sites flanking the yeast CEN-ARS and URA3 sequences, facilitating their removal from the plasmid by digestion with NotI. pCZR239 further includes the original multiple cloning site of pMAL-c2.) The linker was prepared from 100 pmoles each of oligonucleotides zc26,109 (SEQ ID NO:21) and zc26,110 (SEQ ID NO:22) and approximately 5 pmoles each of oligonucleotides zc26,106 (SEQ ID NO:23) and zc26,113 (SEQ ID NO:24). These oligonucleotides were combined by PCR for ten cycles of 94° C. for 30 seconds, 50° C. for 30 seconds, and 72° C. for 30 seconds, followed by a 4° C. soak. The PCR products were concentrated by precipitation with two times the volume of 100% ethanol. The precipitated product was resuspended in 10 μl of water and recombined with 1 μl of SmaI-cut pCZR239 by homologous recombination essentially as described below. The plates were left at room temperature for about 96 hours, then the Ura+ transformants from a single plate were resuspended in 1 ml H₂O and spun briefly to pellet the yeast cells. The cell pellet was resuspended in 1 ml of lysis buffer (2% Triton X-100, 1% SDS, 100 mM NaCl, 10 mM Tris, pH 8.0, 1 mM EDTA). Five hundred microliters of the lysis mixture was transferred to a microcentrifuge tube containing 300 μl acid-washed glass beads, and 500 μl phenol-chloroform was added. The tube was vortexed for 1-minute intervals two or three times, followed by a 5-minute spin in a microcentrifuge at maximum speed. Three hundred microliters of the aqueous phase was transferred to a fresh tube, and the DNA was precipitated with 600 μl ethanol (EtOH), followed by centrifugation for 10 minutes at 4° C. The DNA pellet was resuspended in 100 μl H₂O. 1 μl of the DNA suspension was transformed into E. coli MC1061 (Casadaban et. al. J. Mol. Biol. 138:179-207, 1980) as described below. Clones were screened by colony PCR using 20 pmoles each of oligonucleotides zc26,110 (SEQ ID NO:22) and zc26,109 (SEQ ID NO:21) and 10 μl of an LB-colony mixture as template. The reaction was run for 25 cycles of 94° C. for 30 seconds, 50° C. for 30 seconds, and 72° C. for 30 seconds, followed by a 4° C. soak. Clones displaying the correct size band on an agarose gel were subject to sequence analysis. The correct construct was designated pTAP168.

Plasmid pTAP186 was generated by inserting a PCR-generated linker fragment into the SmaI site of pTAP168 by homologous recombination. The linker was prepared from 100 pmoles each of oligonucleotides zc26,110 (SEQ ID NO:22) and zc26,547 (SEQ ID NO:25) and approximately 5 pmoles each of oligonucleotides zc27,864 (SEQ ID NO:26) and zc27,865 (SEQ ID NO:27). These oligonucleotides were combined by PCR for ten cycles of 94° C. for 30 seconds, 50° C. for 30 seconds, and 72° C. for 30 seconds, followed by a 4° C. soak. PCR products were concentrated by precipitation with two times the volume of 100% EtOH. The precipitated product was resuspended in 10 μl of water and recombined with 1 μl of SmaI-cut pTAP168 by homologous recombination essentially as described below. The plates were left at room temperature for about 96 hours, then the Ura+ transformants from a single plate were resuspended in 1 ml H₂O and spun briefly to pellet the cells. The cell pellet was resuspended in 1 ml of lysis buffer, and the DNA was recovered and transformed into E. coli MC1061 essentially as disclosed above. One clone was screened by PCR as disclosed above using 20 pmoles each of oligonucleotides zc26,547 (SEQ ID NO:25) and zc26,110 (SEQ ID NO:22). Clones displaying the correct size band on an agarose gel were sequenced. The correct construct was designated pTAP186.

Plasmid pTAP238 was generated by inserting a PCR-generated linker into the SmaI site of pTAP186 by homologous recombination. The linker was prepared from 100 pmoles each of oligonucleotides zc29,740 (SEQ ID NO:28) and zc29,741 (SEQ ID NO:29), and approximately 5 pmoles each of oligonucleotides zc29,737 (SEQ ID NO:30) and zc29,735 (SEQ ID NO:31). These oligonucleotides were combined by PCR for ten cycles of 94° C. for 30 seconds, 50° C. for 30 seconds and 72° C. for 30 seconds, followed by 4° C. soak. The resulting PCR products were concentrated by precipitation with two times the volume of 100% ethanol. The precipitated products were resuspended in 10 μl of water and recombined with 1 μl of SmaI-cut pTAP186 by homologous recombination essentially as described below. The yeast cells were plated and left at room temperature for about 72 hours, then the Ura+ transformants from a single plate were resuspended in 1 ml H₂O and spun briefly to pellet the yeast cells. The cell pellet was resuspended in 0.5 ml of lysis buffer. DNA was recovered and transformed into E. coli MC1061 as disclosed above. Clones were screened by colony PCR as disclosed above using 20 pmoles each of oligonucleotides zc29,740 (SEQ ID NO:28) and zc29,741 (SEQ ID NO:29). Clones displaying the correct size band on an agarose gel were subject to sequence analysis. The correct plasmid was designated pTAP238.

One hundred microliters of competent yeast cells (S. cerevisiae) were combined with 10 μl of a mixture containing approximately 1 μl of each of the human zvegf4 fragments (PCR products) and 100 ng of SmaI-digested pTAP238 vector, and transferred to a 0.2-cm electroporation cuvette. The yeast/DNA mixture was electropulsed using instrument settings of 0.75 kV (5 kV/cm), infinite ohms, 25 μF, then 600 μl of 1.2 M sorbitol was added to the cuvette. The yeast was then plated in two 300-μl aliquots onto two -URA D (glucose-containing media lacking uracil) plates and incubated at 30° C. After about 48 hours, the Ura+ yeast transformants from a single plate were resuspended in 1 ml H₂O and spun briefly to pellet the cells. The cell pellet was resuspended in 1 ml of lysis buffer. DNA was recovered as disclosed above. The DNA pellet was resuspended in 100 μl H₂O.

Forty μl of electrocompetent E. coli MC1061 cells were transformed with 1 μl of the yeast DNA. The cells were electropulsed at 2.0 kV, 25 μF and 400 ohms. Following electroporation, 0.6 ml SOC (2% BACTO Tryptone (Difco, Detroit, Mich.), 0.5% yeast extract (Difco), 10 mM NaCl, 2.5 mM KCl, 10 mM MgCl₂, 10 mM MgSO₄, 20 mM glucose) was added to the cells. The cells were allowed to recover at 37° C. for one hour, then were plated in one aliquot on LB+kanamycin plates (LB broth (Lennox), 1.8% BACTO Agar (Difco), 30 mg/L kanamycin).

Individual clones harboring the correct expression construct for human zvegf4-sc-GFD were identified by diagnostic digest of the plasmid DNA. Cells were grown in Super Broth II (Becton Dickinson) with 30 μg/ml of kanamycin overnight. The next day, the cells were harvested, and plasmid DNA was prepared using spin columns (QIAPREP Spin Miniprep Kit; Qiagen Inc., Valencia, Calif.). The DNA was then cut with NotI and XbaI. The clones with the correct restriction pattern were designated pTAP339 and sequenced. The polynucleotide sequence of single chain zvegf4 in pTAP339 is shown in SEQ ID NO:32.

Ten microliters of pTAP339 was cut with 2 microliters of NotI in 3 μl of a commercially available buffer (buffer 3; New England Biolabs) and 15 pl H₂O for one hour at 37° C. 7 μl of the reaction mixture was combined with 2 microliters of 5× T4 DNA ligase buffer (Life Technologies, Gaithersburg, Md.) and 1 microliter of T4 DNA ligase and incubated at room termperature for one hour. One microliter of the ligation mixture was used to transform E. coli strain W3110 (ATCC 27325). The cells were electropulsed at 2.0 kV, 25 μF, and 400 ohms. Following electroporation, 0.6 ml SOC was added to the cells. The cells were grown at 37° C. for one hour, then plated in one aliquot on LB+kanamycin plates.

Individual colonies were picked and grown. Plasmid DNA was prepared using spin columns. The DNA was cut diagnostically with PvuII and HindIII to confirm the loss of yeast URA3 and CEN/ARS elements. This DNA was then transformed into E. coli ROSETTA competent cells (a BL21 derivative with pRARE; obtained from Novagen, Inc., Madison, Wis.) as described above. The cells were plated on 30 μg/ml kanaymcin and 35 μg/ml chloramphenicol due to the presence of pRARE.

An individual colony was picked. Cells were grown in SUPERBROTH II (Becton Dickinson) containing 30 μg/ml of kanamycin and 35 μg/ml chloramphenicol overnight. 100 μl of the overnight culture was used to inoculate 2 ml of fresh SUPERBROTH II containing 30 μg/ml kanamycin and 35 μg/ml chloramphenicol. Cultures were grown at 37° C. with shaking for about 2 hours in 15-ml conical tubes. One ml of the culture was induced with 1 mM IPTG. 2.25 hours later an equal volume of culture was mixed with 250 μl Thorner buffer (8M urea, 100 mM Tris pH7.0, 10% glycerol, 2 mM EDTA, 5% SDS) with 5% βME and dye. Samples were boiled for 5 minutes. Twenty-μl samples were loaded on a 4%-12% PAGE gel (NOVEX). Gels were run in 1× MES buffer. Expression was analyzed by Western blotting.

A NUPAGE 4-12% Bis Tris gel (Invitrogen) was run using 1× MES buffer. 2.5 μl of culture was loaded per lane (5 μl of culture and buffer). A zvegf4 standard (designated “A447F”) was loaded as 25 ng and 50 ng. After the gel was run, the DNA was transferred to a nitrocellulose membrane via a NOVEX transfer box and protocol. The membrane was then blocked in 5% milk and TTBS (160 mM NaCl, 0.1% Tween 20, 20 mM Tris pH7.4) for 30 minutes. It was then incubated at room temperature with an anti-zvegf4 monoclonal antibody (designated E3501) as a 1:5000 dilution. The blot was then washed twice in TTBS for 5-10 minutes each. The washed blot was incubated at room temperature for one hour in a 1:5000 dilution of goat anti-mouse antibody (Bio-Rad Laboratories, Hercules, Calif.). The blot was then washed again in TTBS under the same conditions. The washed blot was then exposed to ECL reagent (Amersham) and exposed to film. Expression was seen in both uninduced and induced samples, presumably due to the known leakiness of the tac promoter. 

1. A polynucleotide encoding a fusion protein consisting of, from amino terminus to carboxyl terminus: a first PDGF-D growth factor domain polypeptide; a linker polypeptide; and a second PDGF-D growth factor domain polypeptide, wherein each of said first and second PDGF-D growth factor domain polypeptides consists of a sequence of amino acid residues as shown in SEQ ID NO:2 or SEQ ID NO:4 from amino acid x to amino acid y, wherein x is an integer from 246 to 258, inclusive, and y is an integer from 365 to 370, inclusive; and wherein said linker polypeptide consists of from 11-40 amino acid residues.
 2. The polynucleotide of claim 1 which is DNA.
 3. An expression vector comprising the following operably linked elements: a transcription promoter; a DNA polynucleotide of claim 2; and a transcription terminator.
 4. The expression vector of claim 3 wherein the DNA polynucleotide further encodes a secretory peptide operably linked to the fusion protein.
 5. The expression vector of claim 3 wherein y is
 370. 6. The expression vector of claim 3 wherein x is 246, 248, or
 250. 7. The expression vector of claim 3 wherein x is 250 and y is
 370. 8. The expression vector of claim 3 wherein the linker polypeptide consists of from 12 to 20 amino acid residues.
 9. The expression vector of claim 3 wherein the linker polypeptide consists of from 14 to 16 amino acid residues.
 10. The expression vector of claim 3 wherein the linker polypeptide does not contain Lys or Arg.
 11. The expression vector of claim 3 wherein the linker polypeptide does not contain Cys.
 12. The expression vector of claim 3 wherein the linker polypeptide does not contain Pro.
 13. The expression vector of claim 3 wherein the linker polypeptide comprises a proteolytic cleavage site.
 14. The expression vector of claim 13 wherein the cleavage site is a plasmin cleavage site, a thrombin cleavage site, or a factor Xa cleavage site.
 15. The expression vector of claim 3 wherein the linker polypeptide comprises the amino acid sequence Ser-Gly-Ser-Gly-Ser-Ser-Gly-Ser-Gly-Ser (SEQ ID NO:33) and a proteolytic cleavage site.
 16. A cultured cell into which has been introduced the expression vector of claim
 3. 17. The cultured cell of claim 16 wherein the DNA polynucleotide further encodes a secretory peptide operably linked to the fusion protein.
 18. A method of making a protein comprising the steps of: culturing the cell of claim 16 in a culture medium whereby the DNA polynucleotide is expressed and the fusion protein is produced; and recovering the fusion protein.
 19. The method of claim 18 wherein the cell is a eukaryotic cell, the DNA polynucleotide further encodes a secretory peptide operably linked to the fusion protein, and the fusion protein is secreted from the cell and is recovered from the culture medium.
 20. The method of claim 19 wherein the linker polypeptide comprises a proteolytic cleavage site and, subsequent to the recovering step, the fusion protein is proteolytically cleaved at the cleavage site.
 21. The method of claim 19 wherein the linker polypeptide comprises a proteolytic cleavage site and the cell produces a protease that cleaves at said cleavage site, and wherein the fusion protein is cleaved by the protease within the cell during secretion. 